We present a protocol that allows us to obtain the concurrence of any two qubit pure state by performing a minimal and optimal tomography of one of the subsystems through measuring a single observable of an ancillary four dimensional qudit. An implementation for a system of trapped ions is also proposed, which can be achieved with present day experimental techniques.
Experimental Determination of Entanglement for Arbitrary Pure States
本文提出了一种实验上确定任意多体纯态局域观测量期望值的方法。在双量子比特系统的实验中，我们采用了对一个态的两个拷贝进行联合测量的方法[S。P. Walborn et al. Nature（London）440，20（2006）]，对于任意维的多体系统，我们在每次测量中只需要一个状态的副本，避免了准备孪生状态或状态的不完美副本。
We consider the distribution of high-dimensional entangled states to multiple parties via noisy channels and the subsequent probabilistic conversion of these states to desired target states using stochastic local operations and classical communication. We show that such state-conversion protocols can be enhanced by embedded channel-estimation routines at no additional cost in terms of the number of copies of the distributed states. The defining characteristic of our strategy is the use of those copies for which the conversion was unsuccessful for the estimation of the noise, thus allowing one …
Quantum entanglement is essential to the development of quantum computation, communications, and technology. The controlled SWAP test, widely used for state comparison, can be adapted to an efficient and useful test for entanglement of a pure state. Here we show that the test can evidence the presence of entanglement (and further, genuine n-qubit entanglement), can distinguish entanglement classes, and that the concurrence of a two-qubit state is related to the test's output probabilities. We also propose a multipartite measure of entanglement that acts similarly for n-qubit states. The number…
Reexamination of determinant-based separability test for two qubits
它在[Augusiak et al.，\; Phys. Rev. A \textbf{77}，030301（R）（2008）]中提出了一种新的量子纠缠度量方法，即通过测量一个可观测量的四个拷贝来区分两个量子比特态的纠缠和可分性，并提出了一种伪纠缠单调函数$π$来量化两个量子比特态的纠缠.本论文的主要目的是显示$π$和并发报告之间的密切关系，有一个结果，共享相同的基本结构的自旋翻转矩阵。我们还表明纠缠的一夫一妻制可以用$π$来重新表述，并证明了$π$的因式分解定律。
We demonstrate to what extent many copies of maximally entangled two-qubit states enable for generating a greater amount of certified randomness than that can be certified from a single copy. Although it appears that greater the dimension of the system implies a higher amount of randomness, the non-triviality lies in the device-independent simultaneous certification of generated randomness from many copies of entangled states. This is because, most of the two-outcome Bell inequalities (viz., Clauser-Horne-Shimony-Holt, Elegant, or Chain Bell inequality) are optimized for a single copy of two-q…
Five-Dimensional Compatibility and Stratified Local-Unitary Structure of Pure Three-Qubit States
School of Physics and Materials Engineering, Hefei Normal University（合肥师范学院物理与材料工程学院）；School of Physics, Anhui University（安徽大学物理学院）；Leibniz International Joint Research Center of Materials Sciences of Anhui Province, Anhui University（安徽大学安徽省材料科学 Leibniz 国际联合研究中心）
We study quantum-limited 3D magnetometry using two qubits. Two qubits form the smallest multi-qubit system for 3D magnetometry, the simultaneous estimation of three phases, as it is impossible with a single qubit. We provide an analytical expression for the Holevo-Cramér-Rao bound (HCRB),the fundamental attainable quantum bound of multiparameter estimation, for 3D magnetometry using two-qubit pure states and show its attainability by rank-1 projective measurements. We also examine the attainability of the HCRB in the presence of dephasing noise using numerical methods. While attaining the HCRB…
Finding the optimal attainable precisions in quantum multiparameter metrology is a non trivial problem. One approach to tackling this problem involves the computation of bounds which impose limits on how accurately we can estimate certain physical quantities. One such bound is the Holevo Cramer Rao bound on the trace of the mean squared error matrix. The Holevo bound is an asymptotically achievable bound when one allows for any measurement strategy, including collective measurements on many copies of the probe. In this work we introduce a tighter bound for estimating multiple parameters simult…
Enhanced measurements on quantum computers via the simultaneous probing of non-commuting Pauli operators
University of Exeter（埃克塞特大学）；Institute for Quantum Computing, University of Waterloo（滑铁卢大学量子计算研究所）；Department of Combinatorics & Optimization, University of Waterloo（滑铁卢大学组合优化系）
As small quantum computers are becoming available on different physical platforms, a benchmarking task known as cross-platform verification has been proposed that aims to estimate the fidelity of states prepared on two quantum computers. This task is fundamentally distributed, as no quantum communication can be performed between the two physical platforms due to hardware constraints, which prohibits a joint SWAP test. In this paper we settle the sample complexity of this task across all measurement and communication settings. The essence of the task, which we call distributed quantum inner pro…
Informationally Complete Distributed Metrology Without a Shared Reference Frame
University of Science and Technology of China（中国科学技术大学）；Anhui Province Key Laboratory of Quantum Network（安徽省量子网络重点实验室）；CAS Center For Excellence in Quantum Information and Quantum Physics（中国科学院量子信息与量子物理学前沿卓越创新中心）；Hefei National Laboratory（合肥国家实验室）
